The present invention relates to a system for assisting in the regeneration of depollution means associated with oxidation catalyst-forming means and integrated in an exhaust line of a motor vehicle diesel engine.
More particularly, the invention relates to a system in which the engine is associated with common manifold or “rail” means for feeding the cylinders of the engine with fuel and adapted, at constant torque, to implement a strategy of regeneration by injecting fuel into the cylinders in at least one post-injection operation.
During regeneration of depollution means such as, for example, a particle filter, stages during which the vehicle accelerator pedal is being raised (no fuel injection in normal operation), and stages during which the engine is idling (exhaust temperature very low), are problematic because they cause the temperature of the exhaust to drop, i.e. the temperature of the exhaust line and of the elements integrated therein.
The use of one or more post-injections during such stages in the lifetime of the engine can serve to limit the drop of temperature in the exhaust line, by relying on catalytic conversion of the hydrocarbons (HCs) produced by the combustion of the post-injection(s) in the engine.
Nevertheless, such strategies rely on the catalyst-forming means reacting in exothermic manner, where said means comprise, for example, an oxidation catalyst or a NOx trap with a function of oxidizing CO and HC, said means being assumed to be active.
During stages in which the engine is returning to idling, as a result of the accelerator pedal being raised, there is no main injection nor any pilot injection, so the or each post-injection does not burn in the cylinder since post-injection serves merely to vaporize fuel in the form of HCs which are converted by the catalyst-forming means.
The temperature at the inlet to the oxidation catalyst-forming means is thus very low and in spite of the exothermic nature of the catalytic reaction produced by the combustion of the HCs derived from the or each post-injection, the front face of the catalyst-forming means cools down progressively so its conversion activity becomes progressively deactivated.
During stages in which the engine is idling, in spite of using one or more post-injections, the temperature at the inlet of the catalyst-forming means is relatively low. The strategy of post-injection while idling also relies on catalyst conversion of the HCs produced by combustion of the post-injection(s) in the engine. In spite of the exothermic nature of the catalytic reaction, the front face of the catalyst-forming means cools progressively and its conversion activity becomes progressively deactivated.
During a stage of prolonged idling, it can happen that the catalyst-forming means are therefore no longer sufficiently active to convert all of the HCs, which leads to peaks of HCs downstream from the catalyst-forming means, or even to blue smoke and/or exhaust odors.
Furthermore, the use of post-injections leads to the fuel diluting the lubricating oil, thereby degrading its lubricating properties, and in particular reducing its viscosity, and if the viscosity becomes too low, that can lead to failure of the engine.